The invention relates to a contact arrangement for vacuum switches with contacts which are movable relative to each other in their axial direction, and in which one of the contacts is designed as a pot contact whose sidewall partly surrounds the other contact. The inner height of the contacts is greater than the opening stroke of the contacts.
Upon opening the contacts of vacuum switches which are subject to a power load an arc is ignited during the separation of the contacts. The arc originates at the extreme end of the metallic point of contact and it spreads to fill the gap between the contacts. The metal vapor required for power conduction is generated in a multiplicity of cathode points which form on the negative electrode. In order to prevent precipitation of the erosion products (metal vapor and metal particles) on the insulator of the switch housing, the quenching chamber of larger-sized vacuum switches is generally provided with at least one so-called condensation screen. The screen, however, makes the switch more expensive and the quenching chamber correspondingly larger.
When currents up to about 10 KA are involved, a diffused metal vapor arc is obtained whose operating voltage is only about 20 to 50 V because of the low field strength in the plasma. The arc stress of such contacts, between which a diffused arc is burning, is therefore relatively low during the quenching process. The task of switching tubes for relatively low switching capacities can therefore be performed by disc contacts with a flat or slightly curved contact gaps. Also, the metal vapor is predominantly emitted in radial direction to the contact surface and the erosion products reach the quenching chamber housing via the shortest route. These erosion products deposit in part in a relatively narrow, annular wall zone concentrically surrounding the gap between the open contacts. Consequently, the thermal stress of this zone is correspondingly high. With increasing thickness of the precipitation layer there is the danger that parts of the layer will detach due to thermal stresses rendering the vacuum switch unusuable. Another portion of the metal vapor escaping from the contact gap is reflected at least once by the housing wall and thus can also reach switch parts not in the direct metal vapor path.
It is known to design one of the two coaxially disposed contacts of vacuum switches with so-called axial field contacts as recessed contacts whose inner sidewall, expands conically towards the rim and overlaps the other contact whose outside also expands conically. The opening angle of the recessed contact is selected smaller than the opening angle of the other contact, resulting in an annular contact surface between the conical surface parts when the contacts are closed and in the ignition of an arc when the contacts are opened. This design is supposed to cause the arc to rotate in a radial plane between the conical surfaces through the action of magnetic force (German disclosure No. 29 25 189).
On the other hand, in a contact arrangement of the kind described at the outset with so-called flat contacts, the contact surfaces are formed in a plane, central area at the bottom of the recessed contact and at the free face of the other contact.
Such a known contact arrangement for vacuum switches contains recessed contacts where the sidewall of the one contact whose contact bottom serves as contact surface surrounds, at least in part, the other contact whose face forms the contact surface. The arc is driven by magnetic forces radially outward between the concentric sidewalls of the contacts of which mutually facing surfaces serve as burn-up areas (German disclosure No. 25 46 376). Accordingly, in these embodiments, the erosion products of the arc can spread almost unhindered into the switch housing.